1. Field of the Invention
The present invention relates to multi-channel audio reproducing devices, and more particularly to a DVD audio reproducing device of a type reproducing DVD audio, that is, super hi-fi (High Fidelity) multi-channel audio recorded on a DVD.
2. Description of the Background Art
A digital versatile disc (DVD) is a recording medium originally developed for video, the capacity of which is more than six times as large as a compact disc (CD). Recording only audio on such large-capacity DVD allows multi-channel reproduction of super hi-fi audio.
By taking the CD as an example, 16-bit linear quantization is carried out at a sampling frequency of 44.1 kHz, realizing two-channel reproduction of audio having a reproduction frequency up to 22 kHz and a dynamic range of 98 dB. On the other hand, as to the DVD audio, by 24-bit linear quantization carried out at the sampling frequency of 96 kHz, for example, six-channel reproduction of audio having the reproduction frequency of 48 kHz and the dynamic range of 144 dB can be realized.
Quality and fidelity of reproduced audio generally mean a level of the upper limit of the reproduction frequency, the dynamic range, and flatness of the frequency characteristics. Hereinafter, the quality and fidelity of reproduced audio only mean the level of the upper limit of the reproduction frequency.
Standards of the DVD audio are still being defined for the practical use. Therefore, there is no conventional DVD audio reproducing device of a type reproducing the DVD audio. Accordingly, described below is a conventional DVD reproducing device used to reproduce video recorded on the DVD and audio accompanying therewith.
FIG. 5 is a block diagram showing the main part of a circuit in the conventional DVD reproducing device, where audio is reproduced.
What is reproduced by the device in FIG. 5 is video data recorded on the DVD and audio data accompanying therewith. The video data and audio data s are separately reproduced in a different processing circuit. It is thus not described herein how the video data is reproduced.
The audio data recorded on the DVD is audio obtained by, in accordance with MPEG2, for example, encoding and modulating digital audio data converted from analog audio picked up by a microphone, for example. The audio is in the form of multi-channel data (in this example, six-channel data).
Referring to FIG. 5, the audio data read from the DVD is provided for a demodulating part 50, and then is demodulated therein. An output of the demodulating part 50 is supplied to an error correcting part 51, then any error of which is corrected therein. An output of the error correcting part 51 is supplied to an audio decoding part 52.
The audio decoding part 52 decodes the provided audio data, and an output thereof is inputted into a DAC part 53.
The DAC part 53 is structured by a DAC (Digital Analog Converter), for example, and converts the inputted audio data in every channel from digital to analog through time-division processing. Alternatively, the DAC part 53 may be structured by three DACs and each DAC may convert the audio data in two channels, or by six DACs on a channel basis. Note that a fewer number of DACs is better in view of the device size and price if the processing speed of the DAC allows such a reduction.
An output of the DAC part 53 is supplied to an LPF part 54.
The LPF part 54 is structured by six low-pass filters (LPFs), and extracts any component equal to or lower than a predetermined frequency (so-called cutoff frequency) from an audio signal in the respective given channels so as to output the same as the 1st channel to 6th channel analog audio signals.
In this manner, the analog audio signals in the respective channels outputted from the conventional DVD reproducing device are amplified in a not-shown amplifier, and then are inputted into six speakers as shown in FIG. 6. Each of the speakers drives a diaphragm in response to the inputted analog audio signals so as to convert the signals into vibrations of air perceivable by user""s ears, and then outputs the same.
FIG. 6 is a schematic diagram showing the structure and placement of a conventional speaker system being connected to the conventional DVD reproducing device in FIG. 5. In FIG. 6, the conventional speaker system includes a pair of main speakers-60, a pair of rear speakers 61, a center speaker 62, and a sub-woofer 63.
Assuming that the user sits at the center of a room, the pair of main speakers 60 are placed toward the front-left and front-right positions of the user, and the pair of rear speakers 61 are placed toward the back-left and back-right positions of the user. Moreover, placed between the pair of main speakers 60 is the center speaker 62, and the sub-woofer 63 is placed in a corner.
The center speaker 62 is a speaker mainly for mid-frequency ranges, and is provided to clearly reproduce vocals (human voice). The sub-woofer 63 is a (non-directional) speaker for low-frequency ranges, and enhances the reproduced audio in the ranges.
As is known from the above, the conventional DVD reproducing device shown in FIG. 5 has the capability of reproducing audio (multi-channel audio data) accompanying the video recorded on the DVD. As the DVD audio is regarded as the multi-channel audio data, the DVD audio reproducing device should be realized, on principle, by utilizing the capability.
Such a DVD audio reproducing device, however, bears the following problem (1).
(1) It is now under study, in the DVD audio specifications, whether or not channels can be different in sampling frequency. If channels vary in sampling frequency, all the best possible 5 use of the large-but-limited DVD capacity is enabled. If this is the case, however, it necessitates a number of the DACs as many as the number of channels. This is because, there is yet no such DAC that converts multi-channel audio data from digital to analog in a time sharing manner while switching the sampling frequency. Further, it is not easy to develop such a DAC. If the device is equipped with a number of the DACs as many as the numbers of channels, size and cost thereof will be enormous.
Moreover, in order to popularize the DVD audio reproducing device, it is important to give consideration to the following issue (2) while pursuing a high-end performance (that is, the attainable superlative performance).
(2) In order to make full use of the high-end performance feasible for the DVD audio, the whole audio system, inclusive of an amplifier and speaker, for example, needs to be high in performance as much as the DVD audio reproducing device. The problem is that, not every user owns such an amplifier or speaker, and these cost a lot together with the DVD audio reproducing device. Accordingly, many users are likely to connect a newly-purchased DVD audio reproducing device to their current or existing audio system for the time being. It is also likely for many users that the high-end performance is not what they are asking for but easy access to DVD audio with their current or existing audio system.
Therefore, it is desirable for the DVD audio reproducing device to be structurally connectable to the existing audio system such as the conventional speaker system shown in FIG. 6. In such case, the DVD audio reproducing device may not be high-end in performance, but can be easily accessible to high quality DVD audio in high quality.
Therefore, an object of the present invention is to provide a DVD audio reproducing device of a type converting DVD audio (multi-channel audio data), from digital to analog, having channels different in sampling frequency by using one or more DACs which are fewer in number than the channels, and then reproducing the converted DVD audio.
Another object of the present invention is to provide a DVD audio reproducing device of a type being structurally connectable to the existing audio system, and being easily accessible to the DVD audio only by connecting thereto.
The present invention has the following features to solve the problem above.
A first aspect of the present invention is directed to a multi-channel audio reproducing device for reproducing multi-channel audio data recorded on a recording medium and obtained by encoding audio data in a plurality of channels sampled at each different sampling frequency, the multi-channel audio reproducing device comprising:
a reading part for reading out the multi-channel audio data from the recording medium;
a decoding part for decoding the multi-channel audio data read by the reading part;
a DAC part for converting, from digital to analog, the audio data in each of the channels obtained after the decoding by the decoding part; and
an LPF part for extracting, from an audio signal in each of the channels obtained after the D/A conversion by the DAC part, any component equal to or lower than a cutoff frequency which is assigned in accordance with a sampling frequency of each of the audio signals, and outputting the same as a analog audio signal for every channel, wherein:
the decoding part includes a sampling frequency converting part for executing sampling frequency conversion with respect to the audio data in at least one channel obtained by decoding so that a sampling frequency thereof coincides with that of the other channels; and
the DAC part includes a digital analog converter converting, from digital to analog in a time sharing manner, the audio data in the plurality of channels equalized in sampling frequency through the sampling frequency conversion.
As described above, in the first aspect, such sampling frequency conversion is executed with respect to the audio data in at least one channel so that the sampling frequency thereof coincides with that of the other channels. In this manner, the audio data in the plurality of channels is equalized in sampling frequency and can be converted from digital to analog in the time sharing manner. Therefore, multi-channel audio data whose channels are different in sampling frequency can be converted, from digital to analog, by using the DACs fewer in number than the channels.
According to a second aspect, in the first aspect:
the LPF part includes variable low-pass filters in as many numbers as the channels of the multi-channel audio data, to which the audio signal in each of the channels obtained after the D/A conversion by the DAC part is inputted, and which have variable filtering characteristics of passing only a component equal to or lower than a cutoff frequency assigned to each of the audio and signals; and
the decoding part includes a control part for controlling the filtering characteristics of each of the variable low-pass filters in relation-to the sampling frequency conversion.
As described above, in the second aspect, such sampling frequency conversion is carried out with respect to the audio data in at least one channel so that the sampling frequency thereof coincides with that of the other channels. Consequently, the sampling frequencies become fewer in selection, and so do the cutoff frequencies. Therefore, by using the low-pass filters having variable filtering characteristics, the filtering characteristics thereof can be controlled in an easier manner.
According to a third aspect, in the first aspect, the LPF part includes low-pass filters in as many numbers as the channels of the multi-channel audio data, to which the audio signal of each of the channels obtained after the D/A conversion by the DAC part is inputted, and which have unchanging filtering characteristics of passing only a component equal to or lower than the cutoff frequency assigned to each of the audio signals.
As described above, in the third aspect, such sampling frequency conversion is carried out with respect to the audio data in at least one channel so that the sampling frequency thereof coincides with that of the other channels. Consequently, the sampling frequencies become fewer in selection, and so do the cutoff frequencies. Therefore, the types of the filters can be reduced by using the low-pass filters having unchanging filtering characteristics.
According to a fourth aspect, in the first aspect, the sampling frequency converting part executes the sampling frequency conversion by interpolation when the sampling frequency is over-sampled, and by decimation when down-sampled.
As described above, in the fourth aspect, the sampling frequency conversion is done by interpolation/decimation. Thereof ore, the sampling frequencies can be converted in an easier manner.
A fifth aspect of the present invention is directed to a DVD audio reproducing device for reproducing multi-channel audio data, recorded on a DVD, obtained by encoding audio data in a plurality of channels sampled at each different sampling frequency, the DVD audio reproducing device comprises:
a reading part for reading out the multi-channel audio data from the DVD;
a decoding part for decoding the multi-channel audio data read by the reading part;
a DAC part for converting, from digital to analog, the audio data in each of the channels obtained after the decoding by the DAC part; and
an LPF part for extracting, from an audio signal in each of the channels obtained after the D/A conversion by the DAC part, any component equal to or lower than a cutoff frequency assigned in accordance with a sampling frequency of each of the audio signals, and outputting the same in the form of analog audio signal for every channel, wherein:
the decoding part includes a sampling frequency converting part for executing sampling frequency conversion with respect to the audio data in at least one channel obtained by decoding so that a sampling frequency thereof coincides with that of the other and channels; and
the DAC part includes a digital analog converter converting, from digital to analog in a time sharing manner, audio data in the plurality of channels equalized in sampling frequency through the sampling frequency conversion.
As described above, in the fifth aspect, the sampling frequency conversion is carried out with respect to the audio data in at least one channel so that the sampling frequency thereof coincides with that of the other channels. In this manner, audio data in a plurality of channels equalized in sampling frequency can be converted from digital to analog in the time sharing manner. As a result, DVD audio (multi-channel audio data) whose channels are different in sampling frequency can be converted from digital to analog by using the DACs fewer in number than the channels.
A sixth aspect of the present invention is directed to a DVD audio reproducing device for reproducing six-channel audio data, recorded on a DVD, obtained by encoding audio data in six channels sampled at each different sampling frequency, the DVD audio reproducing device comprises:
a reading part for reading out the six-channel audio data from the DVD:
a decoding part for decoding the six-channel audio data read from the reading part;
a DAC part for converting, from digital to analog, the audio data in each of the channels obtained after the decoding by the decoding part; and
an LPF part for extracting, from an audio signal in each of the channels obtained after the D/A conversion by the DAC part, any component equal to or lower than a cutoff frequency assigned in accordance with a sampling frequency of each of the audio signals, and outputting the same in the form of analog audio signal for every channel, wherein:
the decoding part includes a sampling frequency converting part for executing sampling frequency conversion with respect to the audio data in three channels obtained by decoding so that each sampling frequency thereof coincides with that of other three channels; and
the DAC part includes three digital analog converters converting, from digital to analog in a time sharing manner, the audio data in two channels equalized in sampling frequency through the sampling frequency conversion.
As described above, in the sixth aspect, the sampling frequency conversion is executed with respect to the audio data in three channels so that each sampling frequency thereof coincides with that of the remaining three channels. In this manner, the two-channel audio data in three pairs equal in sampling frequency can be converted from digital to analog in the time sharing manner. As a result, DVD audio (six-channel audio data) whose channels are different in sampling frequency can be converted from digital to analog by using three DACs, and then reproduced.
Further, the device having a six-channel output is structurally connectable to the existing audio system. Therefore, the device can be connected thereto, and thus DVD audio is easily accessible. In this case, the high-end performance feasible for the DVD audio is not always available, but reproduced audio from the conventional system can be higher in audio quality.
Still further, all of the three DACs are provided with the capability of converting two-channel audio data from digital to analog in the time sharing manner. Therefore, the DAC does not require high-speed processing. Moreover, a recently-popular DAC is the type for the two-channel audio data. Therefore, using the type provides the device cost advantage.
According to a seventh aspect, in the sixth, aspect:
the LPF part includes six variable low-pass filters, to which the audio signal in each of the channels obtained after the D/A conversion by the DAC part is inputted, and which have variable filtering characteristics of passing only a component equal to or lower than a cutoff frequency assigned to each of the audio signals; and
the decoding part, in relation to the sampling frequency conversion, includes a control part for controlling the filtering characteristics of each of the variable low-pass filters.
As described above, in the seventh aspect, the sampling frequency conversion is carried out with respect to the audio data in three channels so that each sampling frequency thereof coincides with that of the remaining three channels. As a result, the sampling frequencies become fewer in selection to three, and so do the cutoff frequencies. Therefore, by using the low-pass filters having variable filtering characteristics, the filtering characteristics can be controlled in a much easier manner.
According to an eighth aspect, in the sixth aspect, the LPF part includes six low-pass filters, to which the audio signal in each of the channels obtained after the D/A conversion by the DAC part is inputted, and which have unchanging filtering characteristics of passing only a component equal to or lower than the cutoff frequency assigned to each of the audio signals.
As described in the above, in the eighth aspect, the sampling frequency conversion is carried out with respect to the audio data in three channels so that each sampling frequency thereof coincides with that of the remaining three channels. As a result, the sampling frequencies become fewer in selection to three, and so do the cutoff frequencies. Therefore, the types of the filters can be reduced to half by using the low-pass filters having unchanging filtering characteristics.
According to a ninth aspect, in the sixth aspect, the sampling frequency converting part executes the sampling frequency conversion by interpolation when the sampling frequency is over-sampled, and by decimation when the sampling frequency is down-sampled.
As described above, in the ninth aspect, the sampling frequency conversion is carried out by interpolation/decimation. Therefore, the sampling frequencies can be converted in an easier manner.
A tenth aspect of the present invention is directed to a method of reproducing multi-channel audio obtained by encoding audio data in a plurality of channels sampled at each different sampling frequency, comprising:
a step of decoding the multi-channel audio data;
a step of converting, from digital to analog, the audio data in each of the channels obtained by decoding by the multi-channel audio data; and
a step of extracting, from an audio signal in each of the channels obtained by decoding the multi-channel audio data, a component equal to or lower than a cutoff frequency assigned in accordance with a sampling frequency of each of the audio signals, and outputting the same in the form of analog audio signal for every channel, wherein:
in the decoding step, sampling frequency conversion is executed with respect to the audio data in at least one channel obtained by decoding so that a sampling frequency thereof coincides with that of the other channels; and
in the D/A conversion step, the audio data in the plurality of channels equalized in sampling frequency through the sampling frequency conversion is converted from digital to analog in a time sharing manner.
An eleventh aspect of the present invention is directed to a recording medium on which a program for reproducing multi-channel audio data obtained by encoding audio data in a plurality of channels sampled at each different sampling frequency is recorded to be run in a computer device, the program for realizing an operational environment comprising:
a step of decoding the multi-channel audio data;
a step of converting, from digital to analog, the audio data in each of the channels obtained by decoding the multi-channel audio data; and
a step of extracting, from an audio signal in each of the channels obtained by decoding the multi-channel audio data, a component equal to or lower than a cutoff frequency assigned in accordance with a sampling frequency of each of the audio signals, and outputting the same in the form of analog audio signal for every channel, wherein:
in the decoding step, sampling frequency conversion is executed with respect to the audio data in at least one channel obtained by decoding so that a sampling frequency thereof coincides with that of the other channels; and
in the D/A conversion step, the audio data in the plurality of channels equalized in sampling frequency is converted from digital to analog in a time sharing manner through the sampling frequency conversion is recorded.